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Old April 6, 2013, 09:07 PM   #1
Cesure
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Poor Man's Chronograph

Has anyone tried measuring bullet velocity with a PC soundcard? I found this PDF when I was searching for low budget chronographs. It details how to measure the time lapse between the sound of a gunshot and the sound of a target hit 50'-150' away. Then if you know the ballistic coefficient of the bullet you can calculate the MV. The theory sounds good, but I wonder if anyone has tested it against a chronograph.
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Old April 6, 2013, 10:13 PM   #2
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I don't know about poor man's... this assumes you have a laptop with a good sound card, a good mic and a pretty good sample rate. Then you'd have to be able to cursor accurately enough over your scope plot to get a good time measure. By the time you get to all that, you could have bought a crony! It'd be a fun experiment though.
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Old April 6, 2013, 10:43 PM   #3
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The F-1 is around $80 new. A decent PC mic will run you $30 and up. Assuming you already have a laptop, you will still need long enough range and steel targets. It would be an interesting exercise but I'd stick to the chrony.
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Old April 7, 2013, 01:29 AM   #4
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Cd quality sampling rate is 41,000 cycles per second.

Sound travels 1,100 fps (1,126 ft/s precisely, but it can vary depending on air density, etc.)

A sound card sampling at 41,000 times per second listening for the impact on a target 90.9 feet away (to make calculations easy).

Suppose a bullet were travelling 1,100 fps. The time between the report of the shot and the sound of the hit would be about 1/6 second. The time between report of the shot and the sound of the hit at 2,200 fps would be 1/8 second. At 3,300 fps about 1/9 second.

At a sampling rage of 41,000 times per second, a single cycle can differentiate
between 1100 fps and 1100.3 fps.
between 2200 fps and 2201.3 fps and
between 3,300 fps and 3,302.9 fps.

That is assuming the microphone and sound card will pick up the exact cycle, which I am guessing might be a bit of a stretch.

Still, the theory is sound.

Now, shorten the distance to 22.75 feet and the resolution changes to:
1100 to 1101.3
2200 to 2205.2
3300 to 3311.7

If the speed of sound varies, (as it does) your calibration will be off.

But if you are going to shoot at a target that clangs, you might consider using a two-microphone setup with one mic downrange and the other at the firing line.


That would eliminate the atmospheric speed of sound variation. Simple two-conductor wire going to the target location with the cheapest microphone you can get, located, say, 4' from the target (right channel of a stereo card laying on the ground with the target 4' tall) and a second mic (left channel) at the firing line 4' from the muzzle.

I think the only trick would be making sure there is no time difference between the sound of firing and the exit of the muzzle (like the blast from the barrel/cylinder gap and the blast from the muzzle).

Not as good as my idea of a radar gun capable of reading the reflection from the base of a departing bullet, but a lot cheaper.

Lost Sheep

p.s. The cheapest Chronographs are under $100. A portable PC, sound card, microphones and a program to read out the velocity rather than you having to read the traces from a sound card, input them into a spreadsheet and find the velocity is well worth $100 and the risk you might shoot your instrument. Besides, you still have to keep your muzzle a precise distance from the target (which must be able to make a decent amount of noise when hit as well as being the precise distance). Using a chronograph is a lot simpler.

Last edited by Lost Sheep; April 7, 2013 at 01:54 AM.
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Old April 7, 2013, 01:39 AM   #5
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If you have to calculate the velocity anyway, you can set up a target at 100 and 200 yards. Zero at 100. Shoot at 200. Measure the drop and plug in the calculations until you have a velocity that matches your drop. Not as precise as a sound card, but less expensive.
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Old April 7, 2013, 09:41 AM   #6
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Well, I've got the laptop and I've got the mic and the file explains how to compensate for the speed of sound and I found the freeware that the article mentions, so my investment so far is zero, but I guess the answer to my question is "No" so far. I might give it a try when the weather gets a little warmer, but I don't have a chrono, cheap or otherwise, to compare it to. At that price, I would be happy with +- 50'.
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Old April 7, 2013, 11:02 AM   #7
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The main problem will be ballistic coefficients. Bryan Litz measured quite a number for his book and found some manufacturers claimed BC's that turned out to be off 10% or more.
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Old April 7, 2013, 11:34 AM   #8
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The bullet velocity will only be averaged over the distance used to measure the peaks, so the velocity won't drop that much if the distance is minimized. and the BC won't be that important. For slower bullets, you can move the measuring distance in to decrease the interval, especially if you have low ambient noise and few nearby sources of echoes from the initial report. For faster bullets, you would need the 150' just to distinguish the target hit from the report, but still, you could probably assume that a 10% error in the BC isn't going to produce a huge error in the calculated MV. I guess it could depend on your definition of huge. I have slow and fast rounds to play with.
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Old April 8, 2013, 01:06 PM   #9
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If you have the time and inclination, I would be interested in hearing how the results compare under the two vastly different methods. Yeah, I'm a geek.
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Old April 8, 2013, 01:16 PM   #10
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A typical sound card for recording is 96K or better. So if you have a really good sound card you can improve things. I have a firewire MOTU sound card that's 96K
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Old April 8, 2013, 01:58 PM   #11
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Quote:
A typical sound card for recording is 96K or better. So if you have a really good sound card you can improve things. I have a firewire MOTU sound card that's 96K
96K what? I have a sound card that came integral to the laptop. I'll be trying that. It worked real well with just two handclaps using the Audacity freeware and the built in mic. I have a better mic to use for the testing. I'm not trying to get .1fps accuracy. I'll be happy with 50fps.
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Old April 8, 2013, 02:43 PM   #12
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Quote:
Well, I've got the laptop and I've got the mic and the file explains how to compensate for the speed of sound and I found the freeware that the article mentions, so my investment so far is zero, but I guess the answer to my question is "No" so far. I might give it a try when the weather gets a little warmer, but I don't have a chrono, cheap or otherwise, to compare it to. At that price, I would be happy with +- 50'.
If you're a computer geek like me, as well as being a gun nut, the OP's idea is simply to make use of something we already have!
Which, if it turns out to be accurate, will be great. But then you'll need a chronograph to check it!
(really just trying to help you justify the Chrony)
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Old April 8, 2013, 02:54 PM   #13
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Quote:
(really just trying to help you justify the Chrony)
Thanks.

I've read about all these people who have chronies that are tempermental, provide suspect readings or end up getting shot, so I just wanted to avoid the downsides of owning one.
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Old April 8, 2013, 08:43 PM   #14
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96K means sampling rate in Khz. The higher the rate the more accurate your method will be.
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Old April 8, 2013, 08:57 PM   #15
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I figured it might be Hz. I'm glad it wasn't 96K$, That would kinda blow the whole poor man's theme.
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Old April 8, 2013, 10:14 PM   #16
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Once you complete this project, be sure to try the ballistic pendulum too!
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Old April 9, 2013, 10:18 AM   #17
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I also thought this thread was going to be about the ballistics pendulum. Imagine my surprise. My technology is still 19th century, I guess. I always knew I was born too late.

You can get guestimates as good simply by looking in the ballistics tables in the back of most reloading manuals.
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Old April 9, 2013, 10:24 AM   #18
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I heard it was a PITA to calibrate a ballistic pendulum.
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Old April 9, 2013, 12:12 PM   #19
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If you shoot .45acp you can just pull the trigger, then watch your second hand until you hear the ping on the steel.
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Old April 9, 2013, 02:46 PM   #20
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Good 'n', Boogie Man!
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Old April 9, 2013, 09:09 PM   #21
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Quote:
Originally Posted by Idaho Spud
My technology is still 19th century, I guess.
Try 18th century. 1742 to be exact. The 19th century saw the Bashforth and Boulangé electromechanical chronographs and some variations come into being.
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Old April 9, 2013, 10:13 PM   #22
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Wikipedia:

Around 1800, the ballistic pendulum was used to measure the momentum of the projectile fired by a gun; dividing the momentum by the projectile mass gives the velocity.

The 19th century was a hundred years long.
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Old April 10, 2013, 09:58 AM   #23
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Idaho Spud,

The ballistic pendulum was invented by Benjamin Robbins, who died in 1751. The main fact you have to keep in mind regarding the Wikipedia is you never know who's been editing it. "About 1800" is just plain sloppy on the editor's part. This Wikipedia entry has it right, though. Scroll down to the history.
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Old April 10, 2013, 01:02 PM   #24
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Quote:
or end up getting shot
Who told on me?????
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